Twin Lakes Research Articles

Effects of hypolimnetic oxygen addition on mercury bioaccumulation in Twin Lakes, Washington, USA

Twin Lakes, located on the Confederated Tribes of the Colville Indian Reservation in eastern Washington, USA, include North Twin Lake (NT) and South Twin Lake (ST). The mesotrophic, dimictic lakes are important recreational fishing sites for both warm-water bass and cold-water trout. To improve summertime cold-water habitat for trout in NT, dissolved oxygen (DO) addition to the hypolimnion, using liquid oxygen as an oxygen gas source, started in 2009. This study assessed mercury (Hg) in the water column, zooplankton and fish, and related water quality parameters, in Twin Lakes from 2009 to 2012. Because methylmercury (MeHg) buildup in lake bottom water is commonly associated with hypolimnetic anoxia, hypolimnetic oxygenation was hypothesized to reduce Hg in bottom waters and biota in NT relative to ST. Oxygen addition led to significantly higher DO (mean hypolimnetic DO: 2–8mg/L versus <1mg/L) and lower MeHg (peak mean hypolimnetic MeHg: 0.05–0.2ng/L versus 0.15–0.4ng/L) in North Twin. In North Twin, years with higher DO (2009 and 2011) exhibited lower MeHg in bottom waters and lower total Hg in zooplankton, inferring a positive linkage between oxygen addition and lower bioaccumulation. However, when comparing between the two lakes, Hg levels were significantly higher in zooplankton (total Hg range: 100–200 versus 50–100μg/kg dry weight) and trout (spring 2010 stocking cohort of eastern brook trout mean total Hg: 74.9 versus 49.9μg/kg wet weight) in NT relative to ST. Lower Hg bioaccumulation in ST compared to NT may be related to bloom dilution in chlorophyll-rich bottom waters, a vertical disconnect between the location of zooplankton and MeHg in the water column, and high binding affinity between sulfide and MeHg in bottom waters.

Hypolimnetic oxygenation in Twin Lakes, WA. Part II: Feeding ecology of a mixed cold- and warmwater fish community

Line-diffuser hypolimnetic oxygenation (HO) was initiated in North Twin Lake, Washington, in 2009 to mitigate loss of coldwater fishery habitat due to temperature–dissolved oxygen “habitat squeeze” and to reduce internal phosphorus cycling. Active tracking, net-captures, and hydroacoustic analyses demonstrated that trout populations rapidly expanded into increased hypolimnetic habitat within the first few years of oxygenation; however, long-term fishery benefits and many basic ecological aspects of HO have yet to be established. Diet and food web analyses indicate significant changes in feeding ecology of principal coldwater fish species in North Twin in 2012 compared to preoxygenation (2005) and to unoxygenated South Twin in 2012. North Twin rainbow trout (Oncorhynchus mykiss) consumed significantly more large-bodied Daphnia during midsummer 2012 than in South Twin, where rainbow trout fed primarily on littoral amphipods. Additionally, relative gut weight for brook trout (Salvelinus fontinalis) in August 2012 was significantly higher in North Twin compared to South Twin, apparently due to increased access to hypolimnetic zooplankton. Golden shiner (Notemigonus crysoleucas) diets also seem to include more zooplankton in oxygenated North Twin. Littoral-focused largemouth bass (Micropterus salmoides) diets were not altered by HO. Observed changes in feeding ecology following HO have significant implications for future fishery management in the Twin Lakes.

Hypolimnetic oxygenation in Twin Lakes, WA. Part I: Distribution and movement of trout

Summer temperature/dissolved oxygen squeeze conditions have negatively impacted coldwater fisheries, including rainbow (Oncorhynchus mykiss) and brook trout (Salvelinus fontinalis), in North and South Twin lakes of northeastern Washington. In 2009, line diffuser hypolimnetic oxygenation (HO) was initiated in North Twin to prevent anoxia and increase summer trout habitat. Suitable trout habitat volume increased from a low of 0% preoxygenation to 100% of the North Twin hypolimnion following HO. To assess if trout utilized the expanded habitat following oxygenation, summer trout movements and habitat utilization were assessed from 2006 through 2012 using various combinations of ultrasonic telemetry, archival tags, gillnetting, and hydroacoustics. Trout utilized North Twin hypolimnetic habitat more frequently compared to pre-HO years and to unoxygenated South Twin, and swim speeds significantly decreased with oxygenation. Chaoborus zooplankton distributions have also changed following HO; water column densities were significantly lower with oxygenation compared to preoxygenation and to unoxygenated South Twin.

Perancanga Sistem Informasi Perpustakaan SMA Negeri 1 Danau Kembar

Library management at SMA country 1 twin lake in this time not yet optimal and not based on computerization. aim from library information system sma country 1 twin lake based on web is a information system that designed to give and optimize operator performance. in give service to library member, in service search book, loaning service and library book return, accessinged according to online. this application planning uses language java with idea netbeans and place to keep database that is mysql. program implementation at require medium shaped computer that integration webserver. implementation result from information system library this is library operator can make recapitulation member report, book total report, fine report, loaning report and return. Keyword: Library Information System, Java, Mysql And Idea Netbeans.

Response of methylmercury, total mercury, iron and manganese to oxygenation of an anoxic hypolimnion in North Twin Lake, Washington

This study evaluated the impact of oxygenation on the accumulation of methylmercury (MeHg), total mercury (Hg), iron (Fe), and manganese (Mn) in the anoxic hypolimnion of North Twin Lake, Washington (mean depth = 9.7 m). Within 8 hours of the start of the oxygenation test, mean hypolimnetic total Fe dropped from 833 to 243 μg/L (58% dissolved), and Mn decreased from 119 to 32 μg/L (100% dissolved). After 17 days of oxygenation, mean hypolimnetic total Hg decreased from 0.77 to 0.58 ng/L, and MeHg decreased from 0.20 to 0.10 ng/L. A month after the end of the oxygenation test, MeHg, total Hg, Fe, and Mn concentrations rebounded to pre-oxygenation levels. Metal dynamics were explained by the differing redox characteristics of Fe (fast oxidation kinetics; reduction at lower redox potential) and Mn (slow oxidation kinetics; reduction at higher redox potential) in natural waters, and the tendency for Fe oxides to co-precipitate with reduced Mn, ionic Hg, and MeHg. From a lake management perspective, the study highlights the role that oxidized metals in surfacial sediments play in retaining Hg in profundal sediments and suggests that sorption characteristics of metal oxides could be exploited to manage Hg bioaccumulation in aquatic biota. The study also suggests that for oxygenation to effectively repress the accumulation of redox-sensitive compounds in the hypolimnion, oxygenation systems should (1) start before anoxic conditions are established; (2) operate continuously through the entire stratified season; and (3) maintain oxygen levels at the sediment–water interface to ensure oxygen penetration into surfacial sediments.

Managing Incentive Dynamics for Collaborative Governance in Land and Ecological Conservation

Public governance often involves policy tools and stakeholders from multiple sectors. How different policy tools are used may affect the chances that the values and interests of diverse stakeholders can be aligned in mutually supportive ways. Drawing on insights from behavioral and cognitive economics, this article uses the case of land and ecological conservation in Twin Lake, Taiwan, to illustrate how various interactive dynamics—hierarchical exclusion and preemptive effects—may affect efforts in land and ecological conservation involving stakeholders from multiple sectors. Such illustrations may inform the choice and sequencing of policy tools for facilitating collaborative governance.

Visualization of temporal change in soundscape power of a Michigan lake habitat over a 4-year period

Soundscape Ecology is an emerging area of science that does not focus on the identification of species in the soundscape but attempts to characterize sounds by organizing them into those produced by biological organisms such as birds, amphibians, insects or mammals; physical environmental factors such as thunder, rainfall or wind; and sounds produced by human entities such as airplanes, automobiles or air conditioners. The soundscape changes throughout the day and throughout the seasons. The soundscape components that create the sound occur at different frequencies. A set of metrics termed soundscape power was computed and visualized to examine the patterns of daily and seasonal change in the soundscape.Automated recorders were used to record soundscape samples every half hour for one minute duration from six sites on an uninhabited island in Twin Lakes located near Cheboygan in Michigan's northern Lower Peninsula. Each recording was divided into 1kHz frequency intervals and visualization tools were used to examine the soundscape power in each interval during 48 half-hour time segments from April–October for four consecutive years. Daily patterns of soundscape power change were also examined during the seven month sample period. To synthesize the data set, three dimensional contour plots were used to visualize day of the year (x), time of day (y) and soundscape power (z) for several frequency intervals. A further synthesis was developed to visualize soundscape change using a Normalized Difference Soundscape Index (NDSI) which is a ratio of low to high frequencies.The visualization of the soundscape revealed discrete patterns in the soundscape including striking changes in the time of the occurrence of dawn and dusk choruses. The patterns in the soundscape were remarkably similar over the four-year investigation. Soundscape power in the lower frequency examined (1–2kHz) was a dominant feature of the soundscape at Twin Lakes and the low frequency soundscape power was negatively correlated with higher frequency sounds.The soundscape power metrics and the visualizations of the soundscape produced in this study should provide a means of rapidly synthesizing large numbers of recordings into meaningful patterns to examine soundscape change. This is especially useful because of the need to develop indices of ecological metrics based on soundscape attributes to assist resource managers in making decisions about ecosystem integrity. Visualization can also be of immense benefit to examine patterns in large soundscape time series data sets that can be produced by automated recording devices.

Isli and Tislit: The First Dual Impact Crater Discovered in Morocco

Isli and Tislit, the Moroccan “Romeo and Juliette” according to a popular legend gave their name to a twin lakes in Imilchil. There we show evidences that these two lakes are impact craters. The proof as follow: 1) the fragments of iron meteorites found on both sides of Isli and Tislit lakes belong to the same parent meteorite and siderite type; 2) the sedimentary formations which are almost tabular in the area of the lakes become tilted with a centripetal sloping all around and towards Isli lake; 3) the circular shape of the Tislit lake is similar to that of the Isli lake; 4) the impact breccias found on the south of the Isli lake contain shocked quartz crystals; 5) the radial fractures of high pressure affect some sedimentary beds. All these observations give evidence of the presence of an impact crater due to the fall of meteorites.

New Paleoearthquake Ages from the Western Garlock Fault: Implications for Regional Earthquake Occurrence in Southern California

Abstract New paleoseismic investigations on the western segment of the Garlock fault at Twin Lakes, California, reveal evidence for up to six surface ruptures in the past ∼5600 years. Calibrated radiocarbon dates from accelerator mass spectrometer analysis of detrital charcoal constrain the timing of three well‐defined events at Twin Lakes to post‐A.D. 1450 (event A), 720–395 B.C. (event G), and 3425–2200 B.C. (event I); two probable events are also constrained to 625–1525 A.D. (event C), and 155 B.C.–A.D. 615 (event E), and a possible additional event to 3425–2200 B.C. and prior to event I. Our findings offer new insights into mid‐ to late‐Holocene behavior of the western and central segments of the Garlock fault, and regional fault interactions. The timing of the most recent event (MRE) on the western segment likely correlates with the MRE at paleoseismic sites on the central segment, suggesting that both segments do sometimes rupture together during large earthquakes. Evidence for events during periods of seismic quiescence on adjacent segments demonstrates that the western and central segments also sometimes rupture independently of one another. The occurrence of event G during a lull in seismic strain release at 2–5 ka on faults in the eastern California shear zone (ECSZ), contrasts with other studies that suggest the Garlock fault ruptures in phase with ECSZ faults. Our data suggest that seismic strain release on the Garlock fault may actually be more in phase with moment release on the Mojave section of the San Andreas fault and the Transverse Ranges faults of the Los Angeles region. Online Material: Two input files written for the computer program OxCal 4.1.7 and two output files from the OxCal analyses. The first input file calibrates and re‐weights probability density functions (PDFs) for radiocarbon ages based on stratigraphic ordering, and the second input file calculates PDFs for earthquake ages based on radiocarbon ages and stratigraphy bracketing the events.

Polarimetric Doppler Radar Observations of Kelvin–Helmholtz Waves in a Winter Storm

Abstract Kelvin–Helmholtz waves were observed by the Twin Lakes, Oklahoma (KTLX), Weather Surveillance Radar-1988 Doppler (WSR-88D); the Norman, Oklahoma (KOUN), polarimetric WSR-88D; and the polarimetric Collaborative Adaptive Sensing of the Atmosphere (CASA) radars on 30 November 2006 during a winter storm in central Oklahoma. The life cycle and structure of the waves are analyzed from the radar data, and the nearby atmospheric conditions are examined. The initial perturbations associated with the waves are first evident only in the radars’ radial velocity fields. As the waves mature, perturbations become discernable in the reflectivity factor Z and spectrum width (SW) fields of both radars, and in the differential reflectivity Zdr and, to a lesser extent, the cross-correlation coefficient ρhv fields of KOUN. As the waves break and begin to dissipate, the perturbations subside. A dual-Doppler analysis is synthesized to examine the kinematic structure of the waves and to relate the polarimetric observations to the kinematics. It is determined that Z and Zdr are enhanced in regions of upward motion (wave crests), and ρhv is reduced in the same vicinity and near the base of the wave circulations. Vertical velocity perturbations transport horizontal momentum upward and downward, inducing horizontal wind perturbations that are approximately 90° out of phase and downstream from their corresponding vertical velocity perturbations. Perturbations in Z, Zdr, and ρhv are observed in the vicinity of wave crests while SW perturbations occur predominately in and just upstream from wave troughs. It is determined that perturbations in the polarimetric variables are a result of the waves modifying local precipitation microphysics. Perturbations in Z and Zdr are hypothesized to be the result of columnar ice crystal generation whereas those in ρhv likely result from the mixing of ice crystals of various shapes and sizes. Perturbations in SW are a result of turbulent motions likely associated with wave breaking and downward advection of a strong shear layer.

TEXTURE AND COMPOSITION OF GARNET AS A GUIDE TO CONTAMINATION OF GRANITOID PLUTONS: AN EXAMPLE FROM THE GOVERNOR LAKE AREA, MEGUMA TERRANE, NOVA SCOTIA

The Governor Lake area, Meguma terrane, Nova Scotia, is underlain mainly by ca. 373 Ma granitoid rocks of the Trafalgar plutonic suite that intruded metasedimentary rocks of the Goldenville and Halifax groups. Garnet is abundant in the Beaverbank Formation of the Goldenville Group and its enclaves in the granitoid rocks. Single crystals of garnet, locally up to 3 cm in diameter, are also present in the Twin Lakes granodiorite and Bog Island Lake tonalite. On the basis of petrographic examination and electron-microprobe data, we have identified four different types of garnet. Type-1 garnet, generally associated with coticule xenoliths, forms small spessartine-rich (Sps 19–74 ) grains concentrated in irregular to planar aggregates. Crystals range from homogeneous (type 1A) to zoned toward a Mn-enriched rim (type 1B). Type-2 garnet is restricted to metasedimentary host rocks, where it forms small, spessartine-rich (Sps 22–71 ) grains zoned from a Mn-rich core to a Mn-poor rim. Type-3 garnet contains abundant metamorphic inclusions ( e.g. , sillimanite) and commonly has an inclusion-rich core with an inclusion-poor rim. Type-3A crystals have a Mn-rich core (Sps 21–52 ), whereas type-3B crystals have a Mn-rich rim (Sps 15–21 ). Type-4 garnet forms large, subhedral to euhedral crystals with abundant inclusions ( e.g. , apatite, plagioclase) of probable igneous origin. Type-4A garnet is spessartine-poor (Sps 4–11 ) and weakly zoned, whereas zoning in type-4B crystals suggests that a Mn-poor core (Sps 8–16 ) was partially resorbed and overgrown by a Mn-rich rim (Sps 11–26 ). Of the various types of garnet identified in the granitic samples, types 1A and 3B are interpreted as orthoxenocrysts, type 1B, as paraxenocrysts, type 3A, as having an orthoxenocrystic core overgrown by an orthomagmatic or paraxenocrystic rim, and types 4A and 4B, as orthomagmatic. The presence of Mn-rich xenocrystic garnet suggests that the Twin Lakes and Bog Island Lake plutons were contaminated by manganiferous rocks, probably derived from the Beaverbank Formation. Incorporation and assimilation of Mn-rich material may also have led to crystallization of orthomagmatic garnet in the granitic magmas.

Largemouth bass consumption demand on hatchery rainbow trout in two Washington lakes

High mortality (84–89%) of stocked rainbow trout (Oncorhynchus mykiss) in Twin Lakes, Washington, has been partially blamed on predation from non-native largemouth bass (Micropterus salmoides). We examined the gut-content of 434 largemouth bass and applied a bioenergetics model to determine the consumption demand on hatchery-released rainbow trout and other prey species in the Twin Lakes. Largemouth bass consumed approximately 4915 (95% CI; 2393–13,452) fall stocked rainbow trout in South Twin. No rainbow trout consumption was observed in North Twin, despite a similar stocking regime. Approximately 6.3% (95% CI; 3–17%) of total fall stocked rainbow trout in South Twin were consumed by largemouth bass. Rainbow trout stocked in the fall ranged from 100 to 160 mm total length and were all subject to predation by large largemouth bass ⩾300 mm. Large largemouth bass consumed rainbow trout that reached up to 210 mm in length and 58% of bass body length. No predation was observed on larger rainbow trout (215–370 mm) stocked during the spring. Smaller largemouth bass ⩽299 mm consumed primarily invertebrates, including crayfish, and did not consume rainbow trout in either lake. During spring and summer in South Twin Lake, large largemouth bass consumed principally golden shiner (Notemigonus crysoleucas) and crayfish. In North Twin Lake, golden shiner constituted most of the larger bass diet for the entire year. Differences in macrophyte distribution, bathymetry, temperature and/or predator-prey demographics likely influenced variability in largemouth bass predation between lakes. Largemouth bass predation did not account for overall rainbow trout mortality.

A Case Study on the Impact of Moisture Variability on Convection Initiation Using Radar Refractivity Retrievals

Abstract A case study illustrating the impact of moisture variability on convection initiation in a synoptically active environment without strong moisture gradients is presented. The preconvective environment on 30 April 2007 nearly satisfied the three conditions for convection initiation: moisture, instability, and a low-level lifting mechanism. However, a sounding analysis showed that a low-level inversion layer and high LFC would prevent convection initiation because the convective updraft velocities required to overcome the convective inhibition (CIN) were much higher than updraft velocities typically observed in convergence zones. Radar refractivity retrievals from the Twin Lakes, Oklahoma (KTLX), Weather Surveillance Radar-1988 Doppler (WSR-88D) showed a moisture pool contributing up to a 2°C increase in dewpoint temperature where the initial storm-scale convergence was observed. The analysis of the storm-relative wind field revealed that the developing storm ingested the higher moisture associated with the moisture pool. Sounding analyses showed that the moisture pool reduced or nearly eliminated CIN, lowered the LFC by about 500 m, and increased CAPE by 2.5 times. Thus, these small-scale moisture changes increased the likelihood of convection initiation within the moisture pool by creating a more favorable thermodynamic environment. The results suggest that refractivity data could improve convection initiation forecasts by assessing moisture variability at finer scales than the current observation network.

Widespread Distribution of the Non-Native Northern Crayfish (Orconectes virilis) in the Columbia River Basin

Abstract We report here the first records of the non-native northern crayfish (Orconectes virilis) in the Columbia River basin within Washington State. Confirmed occurrences include the reservoirs Lake Rufus Woods and Lake Roosevelt on the Columbia River, North Twin Lake in the upper Columbia River basin, Lake Patterson in the Methow River basin, Wapato Lake in the Lake Chelan basin, and Moses Lake in central Washington. Uncertainty remains as to the source, extent of current distribution, and potential impacts of these O. virilis populations. Release of live bait by anglers is a common pathway for crayfish introductions, although stocking for harvest or aquaculture and releases of pets or laboratory animals is equally likely for the introduction of O. virilis to the region. Evaluating potential impacts of O. virilis on native crayfish, fish, and other species in the Columbia River basin will require additional study. These newly discovered O. virilis populations are likely to naturally disperse further t...

Radar Refractivity Retrievals in Oklahoma: Insights into Operational Benefits and Limitations

Abstract The 2007 and 2008 spring refractivity experiments at KTLX investigated the potential utility of high-resolution, near-surface refractivity measurements to operational forecasting. During these experiments, forecasters at the Norman, Oklahoma, National Weather Service Forecast Office (NWSFO) assessed refractivity and scan-to-scan refractivity change fields retrieved from the Weather Surveillance Radar-1988 Doppler weather radar near Oklahoma City—Twin Lakes, Oklahoma (KTLX). Both quantitative and qualitative analysis methods were used to analyze the 41 responses from seven forecasters to a questionnaire designed to measure the impact of refractivity fields on forecast operations. The analysis revealed that forecasts benefited from the refractivity fields on 25% of the days included in the evaluation. In each of these cases, the refractivity fields provided complementary information that somewhat enhanced the forecasters’ capability to analyze the near-surface environment and boosted their confidence in moisture trends. A case in point was the ability to track a retreating dryline after its location was obscured by a weak reflectivity bloom caused by biological scatterers. Forecasters unanimously agreed, however, that the impact of this complementary information on their forecasts was too insignificant to justify its addition as an operational dataset. The applicability of these findings to other NWSFOs may be limited to locations with similar weather situations and access to surface data networks like the Oklahoma Mesonet.

Using stable isotopes and a multiple-source mixing model to evaluate fish dietary niches in a mesotrophic lake

Christensen, D.R. and B.C. Moore. 2009. Using stable isotopes and a multiple-source mixing model to evaluate fish dietary niches in a mesotrophic lake. Lake Reserv. Manage. 25:167–175. We used stable isotope analysis (SIA) of δ13C and δ15N and a multiple-source mixing model to evaluate dietary niches within the Twin Lakes, Washington, fish community to identify potential for exploitative competition, resource partitioning, and predation. The SIA revealed distinct spatial feeding niches; pelagic species were δ13C depleted, while littoral organisms were δ13C enriched. Trophic feeding niches were identified from δ15N enrichment with each successive trophic level. We found δ15N progression with increasing largemouth bass size, suggesting ontogenetic diet variability. Model results suggested that the smaller bass (i.e., ⩽299 mm length) fed principally on golden shiner, crayfish, and macroinvertebrates while larger bass (⩾300 mm) primarily consumed brook trout and golden shiner. Isotopic signatures for golden shiner were intermediary, indicating both pelagic and littoral feeding sources, including phantom midges (nocturnal and pelagic) and damselflies (littoral) as principal diet sources, suggesting possible horizontal diel migration in golden shiner. Pelagic zooplankton was indicated as the most important food source for rainbow trout. Food resources appeared to be partitioned among the Twin Lakes fish community, possibly limiting competitive interactions. Piscivory in largemouth bass was focused on golden shiner and brook trout, suggesting possible top-down regulation of lower trophic levels. Stable isotope analysis and multiple-source mixing models are useful tools that can improve lake and fishery management decisions by providing data on food web dynamics such as competition, resource partitioning, and predation in lakes.

Reproductive Abnormalities in Trout from Western U.S. National Parks

AbstractReproductive disruption is well documented in polluted areas, such as below sewage treatment plants, but not in ecologically protected environments, such as national parks. In a majority of subalpine lakes sampled in Rocky Mountain and Glacier National parks, we observed intersex male cutthroat trout Oncorhynchus clarkii and brook trout Salvelinus fontinalis at frequencies of 9‐33%. Intersexuality, one form of reproductive disruption, is the presence of both male and female reproductive structures in the same animal. Male cutthroat trout, rainbow trout O. mykiss, and brook trout from these parks also produced elevated levels of the estrogen‐responsive protein vitellogenin, another indicator of reproductive disruption. We did not find reproductive abnormalities in national parks of the Sierra Nevada, Cascades, Olympics, Brooks, or Alaska ranges. To determine whether gonad abnormalities were evident in mountain ecosystems before the production of organic pollutants, we sampled various species of the family Salmonidae collected from the preorganic pollutant era (pre‐1930s). In these museum collections, we observed intersex male greenback cutthroat trout O. clarkii stomias collected in the late 1800s from Twin Lakes, Colorado, in the Rocky Mountains. Our current results suggest that reproductive disruption is occurring in some national parks. The abnormalities were observed in remote, high‐elevation locations, suggesting that they might have resulted from airborne contaminants. Our observation of intersex greenback cutthroat trout from the late 1800s suggests that organic pollutants are not the only factors inducing reproductive disruption. The causes and population‐level consequences of reproductive disruption in these lakes remain to be determined.

Diet Composition and Overlap in a Mixed Warm- and Coldwater Fish Community

ABSTRACT In Twin Lakes, Washington, illegal introductions of largemouth bass (Micropterus salmoides) and golden shiner (Notemigonus crysoleucas) are feared to be impacting economically important rainbow trout (Oncorhynchus mykiss) and brook trout (Salvelinus fontinalis) populations. We evaluated the stomach contents of 69 golden shiner, 146 rainbow trout, 83 brook trout, and 561 largemouth bass during summer stratification in 2004 and 2005, to determine community diet composition and overlap when food resources were partitioned thermally and spatially. Gut content data revealed some diet overlap but also illustrated distinct resource partitioning among all species but not between salmonids. Rainbow and brook trout had similar pelagic based diets of zooplankton and chaoborids, with high estimated diet overlap. Largemouth bass ≥300 mm was piscivorous and consumed principally golden shiner, with some consumption of rainbow and brook trout during late spring and early fall. Largemouth bass ≤299 mm primarily consumed benthic invertebrates in littoral macrophyte beds. Golden shiner diet contained both littoral and pelagic items, consisting of algae, benthic invertebrates, and zooplankton. Preferential differences in temperature, dissolved oxygen, and habitat as well as species size and ontogeny may all contribute to resource partitioning in the Twin Lakes.

Patterns of prey use by lesser scaup Aythya affinis (Aves) and diet overlap with fishes during spring migration

Recent decline in the lesser scaup Aythya affinis population has been linked to changes in wetland conditions along their spring migration routes. In particular, the use of amphipod prey by lesser scaup has declined in many regions of the upper Midwest U.S.A. and has been linked to expanded fisheries, although empirical data on diet overlap are lacking. To explore patterns of prey use by lesser scaup and diet overlap with fishes, we quantified diets of scaup and fishes during the 2003 and 2004 spring migration in eastern South Dakota, U.S.A. We compared diet overlap between lesser scaup and fishes collected from Twin Lakes, South Dakota—an important stopover location for spring-migrating scaup. Plant seeds occurred in >95% of lesser scaup diets (n = 118) and represented an appreciable amount of consumed biomass (>70%). Gastropods, amphipods, and chironomids were the most abundant invertebrate prey taxa and occurred in 29–34% of lesser scaup diets. Although relatively frequent, these taxa each contributed only 4–27% of the diet by weight. Percent dry mass of amphipods, a preferred prey by lesser scaup, was low (4%) indicating that amphipod availability may be reduced during spring migration. Analysis of fish diets showed that black bullhead Ameiurus melas and yellow perch Perca flavescens had the highest diet overlap with lesser scaup at 94% and 92%, respectively. Moreover, mean size of amphipods and chironomids found in fish diets were significantly larger than that consumed by lesser scaup. Our findings support the notion that amphipod use by spring-migrating lesser scaup has declined and that size-selective predation by fishes may influence prey availability for lesser scaup.

Differential Prey Selectivity of Largemouth Bass Functional Feeding Groups in Twin Lakes, Washington

Largemouth bass (Micropterus salmoides) can be top-down regulators in a fish community. It is important for fisheries biologists who manage predator-prey populations to understand when bass become piscivorous. We examined the stomach contents of 622 largemouth bass in watershield (Brasenia schreberi) beds in North and South Twin Lakes, Washington. Bass displayed temporal and ontogenetic diet variation within and between lakes. Bass <100 mm fed principally on zooplankton and scuds in June and September and on benthic invertebrates, especially midge and damselfly larvae, during July and August. Damselflies, midges and scuds were major diet constituents for bass 100–199 mm. Bass 200–299 mm consumed large numbers of macroinvertebrates but also fed on crawfish and golden shiner (Notemigonus crysoleucus). Overall, the importance of large prey items like golden shiners and crawfish increased while macroinvertebrate importance to bass diets decreased as the fish grew larger. Bass piscivory was focused on golden shiners and was only observed in larger fish >300 mm. Golden shiner consumption was the lowest during June and September when cannibalism, crawfish and trout consumption increased. Temporal and ontogenetic variability in bass diets is most likely due to habitat variability, fish size, and prey availability and size. Manipulation of bass piscivory through slot-length-limits and/or macrophyte removal could be examined as a potential method for controlling nuisance forage fish such as golden shiners.